The American aquatic fauna is different from that of Europe, of course, yet the river otter has the same general preferences as the Eurasian: a Lutra lutra transplanted to the USA would not be noted amongst the river otters for its different diet. There have been several detailed summaries of the various kinds of prey taken by Lontra canadensis (Hansen 2003; Melquist and Dronkert 1987; Toweill and Tabor 1982), and here I will mention examples. All data are based on spraint (scat) analysis and, although no evaluations of this technique are available for the river otter, I will assume that it is subject to the same reservations as is the case for the Eurasian otter.
A classical project was that by Melquist and Hornocker (1983), in rivers in the mountains of Idaho. They found fish remains in 93-100% of their sample of almost 2000 scats, and far fewer invertebrates such as large beetles, dragonfly larvae and freshwater mussels. Birds, mammals and the occasional snake were also represented. The fishes were mostly suckers Catostomus macrocheilus, salmonids of several species (including the migratory kokanee Oncochynchus nerka in autumn, and mountain whitefish Prosopium williamsoni in winter), and sculpins Cottus bairdi, bullheads Ictalurus nebulosus and cyprinids. The large majority of fish were sculpins and mountain whitefish. The authors argued that bullheads were underrepresented in the scats, because they have no scales and the sharp spiny remains are often discarded by the river otters.
In Pennsylvania Serfass et al. (1990) studied more than 400 spraints of river otters in lakes and rivers. They mentioned the same problem of underrepre-sentation of bullheads, and also of freshwater mussels, which are eaten by the otters but leave no trace in the scats. Here, also, fish dominated the scene, occurring in 93% of the spraints, followed by crayfish (Cambarus spp.) in 44%, and some frogs, insects, molluscs and mammals. The fish were mostly the slow and abundant species, such as perchlike sunfishes or centrarchids, and suckers, as well as various cyprinids, perch and a few pike. There were seasonal fluctuations in the otters' diet, which appeared to be driven by the increase in crayfish abundance in summer.
In rivers and lakes at the much higher latitudes of northern Alberta, Reid et al. (1994a) similarly found fish in 92% of the almost 1200 scats analysed. These also were mostly slow ones, such as catastomids (suckers), cyprinids and salmonids, as well as sticklebacks. In winter, when otters had to feed under the ice, their diet was less diverse, and they caught almost only sticklebacks and rather slow cyprinid species. The researchers also found bits of insects (large beetles and dragonfly larvae), snails, birds (ducks), a few frogs and occasionally remains of beaver Castor canadensis. It was not certain whether the snail remains in the otter scats were caught by the otters themselves, or by the fish on which the otters fed.
River otters living along sea coasts, such as along Prince William Sound in Alaska, feed mainly on bottom-dwelling fishes (37% of identified food items), but they also take many invertebrates such as marine snails (33%), bivalves (13%) and crabs (9%) (Bowyer etal. 1994).
In none of these studies was the size of fish taken by otters measured. Overall, however, the diet of the North American river otter, mutatis mutandis, can be described in the same terms as that of the Eurasian otter: the vast majority is fish of virtually all species, with emphasis on the slow-moving and bottom-living ones. These animals also take crayfish and some other invertebrates, and a few other odds and ends such as amphibians and reptiles, birds, insects and small mammals.
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